Tunnel junctions based on interfacial two dimensional ferroelectrics

Van der Waals heterostructures have opened new opportunities to develop atomically thin (opto)electronic devices with a wide range of functionalities. The recent focus on manipulating the interlayer twist angle has led to the observation of out-of-plane room temperature ferroelectricity in twisted rhombohedral bilayers of transition metal dichalcogenides. Here we explore the switching behaviour of sliding ferroelectricity using scanning probe microscopy domain mapping and tunnelling transport measurements. We observe well-pronounced ambipolar switching behaviour in ferroelectric tunnelling junctions with composite ferroelectric/non-polar insulator barriers and support our experimental results with complementary theoretical modelling. Furthermore, we show that the switching behaviour is strongly influenced by the underlying domain structure, allowing the fabrication of diverse ferroelectric tunnelling junction devices with various functionalities. We show that to observe the polarisation reversal, at least one partial dislocation must be present in the device area. This behaviour is drastically different from that of conventional ferroelectric materials, and its understanding is an important milestone for the future development of optoelectronic devices based on sliding ferroelectricity.


Reviewer #2 (Remarks to the Author):
In the manuscript of Y. Gao et al., a tunneling junction based on R-stacking MoS2 was constructed and measured at 1.5 K.The hysteretic conductance curves are analyzed through brief modelling for various ferroelectric domain structures.
Sliding ferroelectricity is a recent breakthrough in the community of ferroelectrics.A tunneling device based on the novel ferroelectric is conceptually novel.However, from the presented results one cannot see the promise of such devices (even in the perfect case, configure 1 in Fig. 2).Details are as follows.
1.The conductance ratio between two polarized states is approximately 10.It is expected to be even smaller at room temperature.The temperature dependence of device performance should be presented, in particular, at room temperature.2. A basic question is, why it is so small.Compared with similar tunneling junctions based on In2Se3 or CIPS, such a result seems less appealing.For materials beyond R-stacking bilayer MoS2, is there any hope to improve the result?Practical measures to enhance the conductance ratio should be discussed, since one cannot see immediate results from ref. 1, 3-6 as cited in page 2, line 2.
3. Page 4, paragraph 2: the contact potential difference is claimed to be 0.26 eV.This value corresponding to the difference between graphene and bulk graphite.For the monolayer and few-layer graphene used in the manuscript, the difference in work function should be much smaller.4. Page 6: the domain wall movement is claimed to be gradual.From literatures, it is known to be quite abrupt, especially for the low frequency sweeping of electric field used both in literatures and this work.Is it possible that the abrupt change is smeared out due to the rough signal? 5.In the model of Fig. 2, the quantum capacitance of monolayer graphene seems to be omitted.Can the author estimate the Fermi level variation during voltage sweeping and justify the omission?6.Only one sample for each type is investigated.Whether the quantitative result is robust should be clarified.

Reviewer #3 (Remarks to the Author):
The manuscript "Tunnel junctions based on interfacial 2D ferroelectrics" by Y. Gao, A. Weston, V. Enaldiev et al. addresses one of the key practical challenges concerning 2Dinterfacial ferroelectrics.These findings would have an immediate impact on the research efforts in the community and will ease the path for development, particularly along the application front.The results are interesting and considerable for publication in Nat.Commun.after a few semi-minor clarifications.

Comments:
The novelty of the current manuscript lies in (1) the efforts of studying the individual ferroelectric domain-based FTJs in the reconstructed superlattice rather than a macroscopic average, which has been the case so far.On the other hand, domain wall motion in such structures has been experimentally demonstrated earlier, even by the same group.A few examples are: Fig. 3  Nature (all are cited in the manuscript).Therefore, regarding ferroelectric switching by sliding, the most appealing aspect of the current manuscript is: (2) the understanding (than the observation itself) it imparts on -what factors influence the interlayer sliding in the ferroelectric bilayers.This is certainly missing the existing literature.But, the description of the relevant phenomena is rather ill-described in the main text (detail in my suggestions).Please note that by no means I am undermining the observation.Will the work be of significance to the field and related fields?-Yes Does the work support the conclusions and claims?-Yes Are there any flaws in the data analysis, interpretation, and conclusions?-The authors have provided adequate qualitative justification for their observations Is the methodology sound?Does the work meet the expected standards in your field?-Yes Specific suggestions: 1. Page4.Use of 'shortcut'.Is it a typo?If not, please explain.2. The DC equivalent of the circuit, used for tunneling current measurement (shown in Fig. 1c), is analogous to gating the ferroelectric MoS2 with respect to top graphene.How would the electrostatic doping affect the inbuild dipole moment in R-MoS2?Does it depolarize the ferroelectric crystal? 3. The theory describing seeding size vs. favorable energy, based on competitive domainwall stiffness, deformation energy, and electrostatic energy, lacks an introduction.No physical description has been given besides citing Ref.29.I believe an intuitive description and reasoning are very much needed to make the current manuscript self-contained and would help increase the comprehensibility among a broader class of readers.4. Domains of type 3-4 (in ref. to Fig. 1) do not show any prominent hysteresis in di/dv (Fig. 3b-c).However, it is contrasting (at least partially) to earlier results.See, for example -Fig.

REVIEWER COMMENTS
To all: We would like to thank the Referees once again for their helpful comments and construcfive crificism during this process.We appreciate the fime it has taken to review our manuscript and believe that the manuscript has been considerably improved as the result of their work.A point-bypoint response to the reviewers' concerns follows.

Reviewer #1 (Remarks to the Author):
The authors explore the switching behavior of sliding ferroelectricity in MoS2 using scanning probe microscopy domain mapping and tunnelling transport measurements and found that the switching behavior is strongly influenced by the underlying domain structure.Here are some comments: In Line 45, "In previous years, various groups have reported observafions of intrinsic 2D in-plane ferroelectricity in materials such as (monolayer) SnTe, and out-of-plane ferroelectricity in (d1T) MoTe2, (1T') WTe2,few-layer (Td) WTe2 and CuInP2S6".2D Materials like In2Se3 with out-of-plane and in-plane polarizafion phase needs to be cited here.
We apologise for this omission, the references [14,15] have now been added to the manuscript.
Can authors show the atomic structures of dislocafions between up and down domains in experiments?Do they have the same structure as dislocafion in region 4? Because when modelling a twist system, the up and down domains are separated by special stacking region rather than a tradifional dislocafion.
The referee is correct, the dislocafions in sliding ferroelectricity are substanfially different from those in tradifional ferroelectrics, and of course Cases 1 and 4 also differ.To make it clear to the readers we have now included atomic schemafics for both dislocafion types in Figure 1d,e.These are obtained using mulfiscale modelling [27] and confirmed using earlier TEM studies [28].
Can the movement of dislocafions after applying voltage be characterized by AFM in region 1 and 2?
Due to the (semi)metallic nature of the top graphene electrode, it is not possible to see the TMDs domain structure underneath.For this reason, we have performed the SPM measurements before the graphene was placed and designed the devices keeping this pre-exisfing domain layout in mind.
In region 2, the authors claimed that with shorter moiré period the opposite polarizafion will be more rigid.However, in Figure 3a, the bias voltage is smaller than that in region 1.Can authors explain the reasons?
The full range of the bias voltage as well as the produced electric field are determined by the overall thickness of the dielectric stack, which is different for each device studied.Also, due to the exponenfial nature of the electrical currents we tend to limit the Vsd to the value where full current reaches 1uA to prevent the effects of local heafing and damage to the devices.For the device in Fig. 1 the hBN thickness was 3L and for the device in Figure 3a,b only 2L which is the reason Vsd ranges differ.
In Figure 3a and Figure 3b, can posifive bias voltage show hysteresis?
We apologies for this omission -due to the limited space we only showed the negafive bias side in the main text.The full data sets have now been added to the Supplementary Figure S4.Due to the transport gap being larger on the posifive side, a complete switching is achieved by the fime a measurable current can be detected at Vsd~0.9 V, and further increase in the Vsd creates no changes in the domain structure where measurable current is present.This is a consequence of a specific hBN dielectric thickness used and the fact that domains are switched by the electric field, but the switching can only be probed when a measurable tunnelling current is present.
The authors theorefically analysis different cases to grow a reversal polarizafion domain with out-ofplane electric field.Can any of the cases be verified in your sample?
We apologise for being unclear; the cases (i), (ii) and (iii) in Fig. 3e are directly related to the interpretafion of the experimental results shown in Fig. 3c -we use these to explain the lack of pronounced switching behaviour.In cases (iv) and (v), we propose other structures that can enable the switching in devices with no or only perfect dislocafions.However, these structures are not feasible within the approach we use due to the presence of the TMD flake edge in the tunnelling region, which will cause high tunnelling currents in the region where no TMDs are present.We have added a sentence to the main text to clarify this.

Reviewer #2 (Remarks to the Author):
In the manuscript of Y. Gao et al., a tunneling juncfion based on R-stacking MoS2 was constructed and measured at 1.5 K.The hysterefic conductance curves are analyzed through brief modelling for various ferroelectric domain structures.
Sliding ferroelectricity is a recent breakthrough in the community of ferroelectrics.A tunneling device based on the novel ferroelectric is conceptually novel.However, from the presented results one cannot see the promise of such devices (even in the perfect case, configure 1 in Fig. 2).Details are as follows.
1.The conductance rafio between two polarized states is approximately 10.It is expected to be even smaller at room temperature.The temperature dependence of device performance should be presented, in parficular, at room temperature.
As the referee suggested we have conducted the measurements at high temperatures (100K and 150K) and can confirm that the switching is present but indeed with a smaller hysteresis.This is a consequence of the close proximity of MoS2 conducfion band edge to the Fermi level of graphite (~0.3 eV), which at elevated temperatures leads to the thermal excitafion and doping of the MoS2 conducfion band.We believe that the presence of this finite electron density in MoS2 screens the ferroelectric potenfial and most likely is the main reason why the hysteresis is smaller.This thermal acfivafion also effecfively reduces the tunnelling distance (the tunnelling now takes place from MoS2 to graphene across hBN barrier only), which is evident in the dramafic increase of the tunnelling currents observed (over two orders of magnitude at 100K) unlike for the temperature-assisted tunnelling through hBN (e.g.hftps://www.nature.com/arficles/srep21168).
Our choice of MoS2 for this study was due to the preceding works [22], where the ferroelectric potenfial for 3R MoS2 was first experimentally measured, which made it the best-understood system to explore FTJ devices.This close alignment is a unique feature of MoS2, as the other TMDs have completely different band alignments with graphite (e.g.DOI: 10.1126/sciadv.1601832and doi.org/10.1038/s41586-019-1402-1).Because of this, other TMDs should have different temperature-dependent behaviour.However, we would prefer to leave other materials out of this manuscript for two main reasons: (1) the key conclusions of this work -the changes in the switching behaviour based on the underlying domain structure -will hold, as these are based on the properfies of the domain walls and (2) these devices are notoriously hard to fabricate which would require up to a year unfil the results on other systems are available.
To address the referee's concern, the temperature dependence, along with the relevant discussions, have been added to the SI Figure S5 and the main text.
2. A basic quesfion is, why it is so small.Compared with similar tunneling juncfions based on In2Se3 or CIPS, such a result seems less appealing.For materials beyond R-stacking bilayer MoS2, is there any hope to improve the result?Pracfical measures to enhance the conductance rafio should be discussed, since one cannot see immediate results from ref. 1, 3-6 as cited in page 2, line 2.
We agree with the referee that the ON/OFF rafio is not as high as that demonstrated for the latest convenfional ferroelectric FTJ devices.We do not opfimise the ON/OFF value -this is the first study of sliding FTJ and at this point the system is not understood well enough to deal with the memory window opfimisafion.Our model (Eq (2) in the main text) predicts the value of the hysteresis to be dependent on several parameters, such as the relafive band alignments of the TMDs, hBN and the contacts, the material thicknesses and the ferroelectric potenfial Δ.We are confident the ON/OFF rafio can be improved in future follow-up studies in a similar manner to what was done for convenfional ferroelectric FTJ over the years.One simple way would be to mulfiply the number of ferroelectric interfaces (as tradifional thin film ferroelectrics are typically few nm thick and, therefore, accumulate a much larger potenfial difference).For instance, Nature 612, 465 (2022) shows that six 3R TMD interfaces show cumulafive polarizafion of 330mV versus 63 meV for the single interface we study in this work.
Instead, in this study, we focus on the switching behaviour itself, which, being drasfically different in the sliding ferroelectrics, certainly needs invesfigafing first to understand the behaviour of this novel system.We also developed a simple model which qualitafively explains the observed behaviour and allows other groups to make educated guesses as to how the ON/OFF rafio can be opfimised in the future.We, however, would like to refrain from speculafions regarding specific pathways to opfimise the ON/OFF rafio as these would not be supported by experimental evidence which other referees may request then.
3. Page 4, paragraph 2: the contact potenfial difference is claimed to be 0.26 eV.This value corresponding to the difference between graphene and bulk graphite.For the monolayer and fewlayer graphene used in the manuscript, the difference in work funcfion should be much smaller.
Published literature has different observafions as to how quickly the work funcfion of graphite saturates with the number of layers, but the majority report that at >10L the bulk values are seen e.g.10.1016/j.diamond.2019.107576or 10.1088/0953-8984/29/3/035003.In our work, the boftom graphite drain is not a few-layer one, but has been selected to be thick (~50 nm or ~150L) and therefore we consider it to be bulk.The clarificafion has been added to the main text.
4. Page 6: the domain wall movement is claimed to be gradual.From literatures, it is known to be quite abrupt, especially for the low frequency sweeping of electric field used both in literatures and this work.Is it possible that the abrupt change is smeared out due to the rough signal?
Current studies, e.g. an excellent work on in-situ switching hftps://www.nature.com/arficles/s41563-023-01595-0#Sec13indeed report abrupt switching for large domains and a gradual process for the smaller ones.We do occasionally see abrupt jumps in the tunnelling curves (see, for instance, SI Fig S5a) but these vary between repefifive sweeps.Most likely this is due to the random nature of the disorder pinning the domain wall propagafion, which in our case leads to mixed behaviour (gradual with some random jumps).
5. In the model of Fig. 2, the quantum capacitance of monolayer graphene seems to be omifted.Can the author esfimate the Fermi level variafion during voltage sweeping and jusfify the omission?
We apologize for this omission, in the new version of SI we took into account the quantum capacitance of graphene in equafions for electric field in MoS2 and hBN, which also slightly modifies the equafion for tunnelling current.This, however, turned out to be a small correcfion which is illustrated in the new Supplementary Fig. S7 where both are presented.
6.Only one sample for each type is invesfigated.Whether the quanfitafive result is robust should be clarified.
We would like to point out that all 5 devices studied fit into the overall understanding of the system developed here, even though some specific configurafions have not been replicated.We have now added one more device to the SI which shows consistent behaviour with the sample in Fig. 3b of main text -we hope this safisfies the referee's comment.
We apologize for these omissions; they have been corrected in the new version.

Reviewer #3 (Remarks to the Author):
The manuscript "Tunnel juncfions based on interfacial 2D ferroelectrics" by Y. Gao, A. Weston, V. Enaldiev et al. addresses one of the key pracfical challenges concerning 2D-interfacial ferroelectrics.These findings would have an immediate impact on the research efforts in the community and will ease the path for development, parficularly along the applicafion front.The results are interesfing and considerable for publicafion in Nat.Commun.after a few semi-minor clarificafions.

Comments:
The novelty of the current manuscript lies in (1) the efforts of studying the individual ferroelectric domain-based FTJs in the reconstructed superlaftice rather than a macroscopic average, which has been the case so far.On the other hand, domain wall mofion in such structures has been experimentally demonstrated earlier, even by the same group.A few examples are: .Therefore, regarding ferroelectric switching by sliding, the most appealing aspect of the current manuscript is: (2) the understanding (than the observafion itself) it imparts on -what factors influence the interlayer sliding in the ferroelectric bilayers.This is certainly missing the exisfing literature.But, the descripfion of the relevant phenomena is rather ill-described in the main text (detail in my suggesfions).Please note that by no means I am undermining the observafion.Will the work be of significance to the field and related fields?-Yes Does the work support the conclusions and claims?-Yes Are there any flaws in the data analysis, interpretafion, and conclusions?-The authors have provided adequate qualitafive jusfificafion for their observafions Is the methodology sound?Does the work meet the expected standards in your field?-Yes We are grateful to the Reviewer for the complements and overall posifive evaluafion of our work.Specific suggesfions: 1. Page4.Use of 'shortcut'.Is it a typo?If not, please explain.
We apologize for the confusion, in this case we mean that connecfing the measurement circuit and sefting Vsd = 0 is equivalent to making an electrical short-circuit, i.e. connecfing source and drain together with a wire.We understand this is not an accurate depicfion of the actual circuit, and have changed the main text to address this.
2. The DC equivalent of the circuit, used for tunneling current measurement (shown in Fig. 1c), is analogous to gafing the ferroelectric MoS2 with respect to top graphene.How would the electrostafic doping affect the inbuild dipole moment in R-MoS2?Does it depolarize the ferroelectric crystal?
Due to the band mismatch between the conducfion band of MoS2 and the graphite Fermi level of 0.3 eV, when a small bias is applied (Vsb<0.8V for the device in Fig. 2) the MoS2 remains insulafing.We therefore treat it like an addifional tunnelling barrier (skewed by the field) in our analysis of both experimental results and modelling.For the larger Vsb values the MoS2 indeed becomes doped, and the direct tunnelling into the bilayer becomes significant.Our earlier studies show that such filling of the conducfion band indeed leads to screening of the ferroelectric dipole, through it persists to finite densifies (Weston et al.Nat.Nanotechnol).
3. The theory describing seeding size vs. favorable energy, based on compefifive domain-wall sfiffness, deformafion energy, and electrostafic energy, lacks an introducfion.No physical descripfion has been given besides cifing Ref.29.I believe an intuifive descripfion and reasoning are very much needed to make the current manuscript self-contained and would help increase the comprehensibility among a broader class of readers.
We agree with the referee that the manuscript will benefit from a more general discussion of the modelling.This have now been included in the main text (see highlighted changes).
4. Domains of type 3-4 (in ref. to Fig. 1) do not show any prominent hysteresis in di/dv (Fig. 3b-c).However, it is contrasfing (at least parfially) to earlier results.See, for example - We fully agree with the referee that in both references, two parfial dislocafions are shown to merge and split again with the cycling of the electric field.One explanafion we have is that when two parfial dislocafions collide, they do not necessarily zip into a perfect dislocafion along the enfire length of the boundary.Indeed, this can be seen in Weston et al.Fig. 1e,f where the resolufion is sufficient to spot several pockets where the two parfial dislocafion have not merged and are tens on nms apart.Upon the field reversal, these pockets serve as seeds for the unzipping, hence the observed behaviour.This is in contrast to our current work, where an already formed perfect dislocafion was selected for the device fabricafion.Most likely we deal with a different case where there is no such "weak spots" along the domain wall to provide the seed.To acknowledge the referee's comment we have added a sentence to the main text. 5. Please consider adding a summarized conclusion at the end.
We agree with the referee that the manuscript reads befter with a conclusion -it has now been added.
6. Usually, such device fabricafion involves various thermal annealing steps, both after tear-and-stack (before lithography) and after lithography.If this is the case, comments in supplementary would be helpful for the work to be followed and reproduced in the future.
We apologize for the omission and agree that such details are important.The annealing steps have now been described in the SI secfion 1. Liang et al. Shear Strain-Induced Two-Dimensional Slip Avalanches in Rhombohedral MoS2, Nano Left. 2023, 23, 15, 7228.Consider cifing it if you find it relevant and useful.